x10 - CM12U (CM11) Clock Accuracy
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Posted by Martin Davies on September 25, 2007, 10:09 am
Please log in for more thread options Here's the situation. I've downloaded a simple sequence of timed events to the CM12U using Activehome. The computer is then disconnected from the interface. To begin with the events work ok but very quickly (a couple of days) the timing of events drifts significantly (it appears the internal clock in the interface is not very accurate?) When you reconnect the interface to Activehome and check the interface time it appears correct - maybe a second or twos difference only - this seems strange since the timing of events may be out by upto an hour or two. I've seen elsewhere information about a mod to the clock in a CM11a but this doesn't appear to be relevant for the CM12U? There appears to be no 80C48 chip in the CM12U. Any help or guidance much appreciated. Thanks... | |||||||||||||||||||||||||
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Posted by Charles Sullivan on September 25, 2007, 12:18 pm
Please log in for more thread options The CM11/CM12 clock is usually kept roughly in sync with the AC line frequency, which can fluctuate. In the USA the power companies tweak the frequency up or down so that AC clocks will be accurate on the average over the long term. When disconnected from AC and running under battery power, the CM11A clocks I've tested tend to lose about 6-8 minutes per day. When you disconnect the CM12U from the PC, you _must_ disconnect the cable at the CM12U end. The circuitry is very susceptible to stray pickup from a dangling cable which can screw up the works. Note: If AC power to the CM12U is interrupted even momentarily, then once power is resumed it sends a request for a clock update to the PC. ActiveHome silently complies with this request. Regards, Charles Sullivan | |||||||||||||||||||||||||
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Posted by Jeff Volp on September 25, 2007, 12:56 pm
Please log in for more thread options >
> The CM11/CM12 clock is usually kept roughly in sync with the AC > line frequency, which can fluctuate. In the USA the power companies > tweak the frequency up or down so that AC clocks will be accurate > on the average over the long term. The powerline frequency may be more accurate than you think. As you say, long term is very precise. Short term can deviate slightly, but peak errors are less than 0.1%, and typically better than 0.025%. The deviation is caused by load variation. Generators have enormous rotating inertia, but sudden increases in load will begin to slow them down. Many power plants use the steam turbine cycle, and it takes a finite amount of time to respond to the increasing load. So there can be instantaneous variations in frequency as the control loops respond. The short-term frequency errors occur on both sides, and long term average is very accurate. Years ago the MIT National Magnet lab had a huge flywheel generator brought in to provide the instantaneous energy necessary for their experiments. It was impossible to pull energy of that magnitude off the grid. Beacon Power is a company using flywheel energy to help stabilize the electric grid. This is a more efficient way to handle instantaneous increases in electrical demand compared with having excess capacity available in a steam boiler. Jeff | |||||||||||||||||||||||||
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Posted by Charles Sullivan on September 25, 2007, 3:43 pm
Please log in for more thread options On Tue, 25 Sep 2007 16:56:18 +0000, Jeff Volp wrote:
>>
>> The CM11/CM12 clock is usually kept roughly in sync with the AC >> line frequency, which can fluctuate. In the USA the power companies >> tweak the frequency up or down so that AC clocks will be accurate >> on the average over the long term. >
> The powerline frequency may be more accurate than you think. As you say, > long term is very precise. Short term can deviate slightly, but peak errors > are less than 0.1%, and typically better than 0.025%. The deviation is > caused by load variation. Generators have enormous rotating inertia, but > sudden increases in load will begin to slow them down. Many power plants > use the steam turbine cycle, and it takes a finite amount of time to respond > to the increasing load. So there can be instantaneous variations in > frequency as the control loops respond. The short-term frequency errors > occur on both sides, and long term average is very accurate. > > Years ago the MIT National Magnet lab had a huge flywheel generator brought > in to provide the instantaneous energy necessary for their experiments. It > was impossible to pull energy of that magnitude off the grid. > > Beacon Power is a company using flywheel energy to help stabilize the > electric grid. This is a more efficient way to handle instantaneous > increases in electrical demand compared with having excess capacity > available in a steam boiler. > > Jeff Thanks for the info Jeff. A CM11A that's doing nothing else seems to keep time within a few seconds of NTP time, but will drift a little when given other tasks (like reading its clock?) so I've never trusted it to accurately reflect the daily frequency accuracy. Regards, Charles Sullivan | |||||||||||||||||||||||||
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Posted by Martin Davies on September 25, 2007, 4:47 pm
Please log in for more thread options Hi -
Thanks for this. Do I understand from your message that the CM11/12 uses the mains supply frequency to regulate its internal clock? If so I may have a problem as we are connected to a private generator supply and the frequency does vary somewhat. Even so the variation in frequency doesn't account for the large errors I'm experiencing with timed events. Other postings (c.f http://www.idobartana.com/hakb/) talk about a modification to the internal clock to improve its accuracy but this doesn't seem to apply to the CM12U as it doesn't have an 80C48 chip. I would appreciate hearing of any ideas for improving clock accuracy? Thanks. | |||||||||||||||||||||||||
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>
> Here's the situation. I've downloaded a simple sequence of timed events to
> the CM12U using Activehome. The computer is then disconnected from the
> interface. To begin with the events work ok but very quickly (a couple of
> days) the timing of events drifts significantly (it appears the internal
> clock in the interface is not very accurate?) When you reconnect the
> interface to Activehome and check the interface time it appears correct -
> maybe a second or twos difference only - this seems strange since the timing
> of events may be out by upto an hour or two.
>
> I've seen elsewhere information about a mod to the clock in a CM11a but this
> doesn't appear to be relevant for the CM12U? There appears to be no 80C48
> chip in the CM12U.
>
> Any help or guidance much appreciated. Thanks...